The nature of cellular controls over responses to steroid hormones has been studied through somatic cell hybrids. Processes studied in these hybrids include induction of tyrosine aminotransferase, prolactin, growth hormone and glutamine synthetase and inhibition of nuclic acid uptake, cell growth and cloning efficiency. Steroid-resistance cell variants have been developed. Characterization and purification of glucocorticoid receptors are underway. Receptors for glucocorticoids, androgens, estrogens and progestins, have been studied for their correlation with treatment and disease in human leukemias and breast cancer. Growth control in cell hybrids has been studied and hybrid membrane proteins analyzed.